Deciphering the Metamorphic History of Mantle Xenoliths

Fragments of Earth’s upper mantle brought to the surface by Eocene alkalic magmas in the Highwood Mountains of Montana, USA, record overprinting by transiting magmas at ~1.8 Ga. Two glimmerite veins that crosscut a harzburgite fragment are made up of mica and datable minerals, including rutile, monazite, apatite, and zircon. The zircons reveal a complicated crystallization history, with core-mantle-rim regions imaged by cathodoluminescence, which have U-Pb dates from 1.7 (rims) to 1.9 Ga (cores). These multiple zircon-crystallization events have distinct trace element contents. Positive Ce anomalies, negative Eu anomalies, rare earth element patterns, and Hf-isotope data suggest an ancient continental crustal origin of the melt that deposited the veins. These events coincide with the collision between two ancient crustal blocks that formed the Great Falls Tectonic Zone and suggest the melt derived from subducted continental crust.